Wireless local area networks (“WLANs”) provide wireless access to network resources within a given coverage area. Users may connect to WLANs using any wireless-enabled device, such as computer desktops and laptops, personal digital assistants (“PDAs”), telephones, digital music players, game consoles, and other portable devices, providing virtually unlimited access to users. Because of their relatively low cost, ease of use, and the mobility that they provide, WLANs have become the preferred technology of choice for network access in homes, offices, and designated areas in airports, meeting rooms, coffee shops, and the like.
Most WLANs available today are governed by wireless protocols that establish the rules for coding, authentication and error detection required to send information over a wireless channel. These wireless protocols include, for example, the IEEE 802.11 family of protocols (e.g., 802.11a/b/g/) that have become ubiquitous across the WLAN market. The common set of protocols enables WLAN equipment to be highly interoperable, thereby providing increased flexibility and connectivity to users.
In the current implementations of IEEE 802.11a/g, for example, data rates of up to 54 Mbps are achievable by employing orthogonal frequency-division multiplexing (“OFDM”) as their modulation scheme. OFDM works by splitting the wireless channel into multiple sub-channels and representing them with orthogonal sub-carriers that are each individually modulated. As a result, information may be divided into multiple symbols that are transmitted in parallel through the sub-channels rather than sequentially through one (very broad) channel. This leads to much longer symbol durations, such that the impact of inter-symbol interference is significantly reduced. With less symbols colliding, there is almost no need for additional measures like costly equalization.
Today OFDM is used as the foundation for several standards, including the digital video broadcasting (“DVB”) standard and the WiMax wireless networking standard, while it is a strong candidate for several upcoming standards, such as for high-rate extensions to third-generation communication systems as well as for fourth-generation mobile communication systems. OFDM is also likely to remain the basis for future extensions of the IEEE 802.11 standards, including the IEEE 802.11n proposal for improved system performance.
The IEEE 802.11 architecture consists of two basic components: mobile stations (“STAs”)—frequently called terminals—and access points (“APs”). Terminals may communicate directly with each other in an “ad-hoc mode” forming an independent basic service set or indirectly via an AP forming an infrastructure basic service set (“BSS”). Several BSSs may be connected via a distribution system (“DS”) forming an extended service set (“ESS”). FIG. 1 illustrates the IEEE 802.11 architecture 100 in infrastructure mode.
The IEEE 802.11 protocols are based on a Medium Access Control (“MAC”) sub-layer, MAC management protocols and services, and several physical layers (“PHY”). A medium access scheme referred to as the Distribution Control Function (“DCF”) employs Carrier Sense Multiple Access with Collision Avoidance (“CSMA/CA”) and binary exponential back-off. STAs refrain from transmitting if they detect the wireless channel or medium (“WM”) occupied.
In addition to this physical carrier sensing, the IEEE 802.11 protocols introduce a virtual carrier sensing mechanism: the network allocation vector (“NAV”). The NAV is a time period in which the WM must be treated as busy even if the physical carrier sensing does not indicate this situation. Stations are not, however, allowed to start transmitting immediately after they discover the WM idle after the NAV time period. They have to sense the WM idle for a deterministic time—the so-called Inter-Frame Space (“IFS”)—before starting their transmission. The length of this interval allows granting prioritized medium access for certain transmissions. The smallest interval is called short IFS (“SIFS”), which is specified for each physical layer.
A two-way handshake between transmitter and receiver preceding the transmission of a data frame may be used to exclusively reserve the WM and set the NAV long enough to complete the desired transmission. This two-way handshake is achieved with a Request to Send/Clear to Send (“RTS/CTS”) frame exchange. The RTS/CTS frame exchange is not mandatory but most commonly used by default if the length of a data packet exceeds a given threshold.
Data packets transmitted according to the IEEE 802.11 protocols are encapsulated in a Physical Layer Protocol Data Unit (“PPDU”) depicted in FIG. 2. The PPDU 200 includes a header 205, referred to as the Physical Layer Convergence Protocol (“PLCP”) header, following a PLCP initial preamble 210. The coded data is encoded in packets in a PLCP service data unit 215, referred to as “PSDU”. Most of the PLCP header constitutes a separate single OFDM symbol 220, denoted SIGNAL, that is transmitted with the most robust combination of BPSK modulation and a convolutional coding rate of R=½. The SERVICE field 225 of the PLCP header 205 together with PSDU 215 form a unit denoted as DATA 230. DATA 230 is transmitted at the data rate described in the RATE field 235 of PCLP header 205 and may constitute multiple OFDM symbols.
The OFDM modulation scheme employs a total bandwidth of 16.25 MHz. This bandwidth is split into 52 sub-carriers, from which 4 sub-carriers are used exclusively as pilots. Therefore, 48 sub-carriers of bandwidth 312.5 MHz each are utilized for data transmission. The data is first convolutionally encoded. The resulting data block is transmitted via all 48 sub-carriers employing the same modulation type. Four modulation types are available for modulating the sub-carriers: BPSK, QPSK, 16-QAM and 64-QAM. The choice of the coding/modulation combination is crucial for the performance of a WLAN, i.e., its throughput, power consumption, error behavior, and so on.
The performance of a WLAN can be increased dramatically by dynamically adapting some or all of the OFDM parameters. Dynamic OFDM, as it is commonly referred, encompasses a family of approaches in which the transmitter adaptively controls the modulation type, the transmit power and/or the coding scheme applied on a per packet and/or per sub-carrier basis, in order to adjust itself in a best possible way to the actual values of the sub-carrier gains.
Dynamic OFDM is based on the observation that the gains of individual sub-carriers vary in time and are also frequency-dependent. Previous work has clearly demonstrated that the performance in terms of throughput, power consumption, error behavior, etc., of an OFDM-based WLAN can be significantly improved by adapting the transmit power and/or modulation type to the current gain of each sub-carrier.
Several different dynamic OFDM strategies can be applied, such as, for example, bit loading and adaptive modulation. Bit loading refers to the case where the transmitter maximizes the sum data rate over all sub-carriers by varying the transmit power and modulation type per sub-carrier. A somewhat simpler scheme to apply is adaptive modulation, in which the transmitter assigns each sub-carrier the same transmit power.
In contrast to these and other dynamic OFDM strategies, current IEEE 802.11-based WLANs may only apply link adaptation, in which the same transmit power and modulation type is applied to all sub-carriers, regardless of their individual gains. Previous work has shown that optimal link adaptation schemes have significantly lower performance than dynamic OFDM schemes that adapt the transmit power and/or the modulation type per sub-carrier.
Such dynamic OFDM schemes, however, have not yet been integrated into WLAN protocols, including the IEEE 802.11 protocols. The performance gain of these dynamic OFDM schemes come at such a system-wide cost that such schemes have not been adopted by the wireless protocols. For example, the transmitter must first be able to accurately estimate the sub-carrier gains before adapting the transmit power and/or modulation type per sub-carrier based on their gains. To do so, the transmitter must know the current state of the wireless channel, which consumes system resources such as time, power, and bandwidth.
Computational resources are also required at the transmitter to generate the dynamic OFDM allocations of transmit power and/or modulation type per sub-carrier. The transmitter has to be equipped with enough computational resources such that the generation time of the dynamic OFDM allocations is sufficiently smaller than the time span during which sub-carrier attenuations change significantly. In addition, the receiver has to be informed of all the transmit power and/or modulation type allocations per sub-carrier, otherwise it cannot decode the data correctly.